# Prime and composite numbers review

CCSS Math: 4.OA.B.4
Review prime and composite numbers, and try some practice problems.

## What are prime numbers?

Prime numbers are numbers with exactly $2$ factors.
A prime number's only factors are $\greenD1$ and the number $\purpleD{\text{itself}}$.
$7$ is an example of a prime number. Its only factors are $\greenD1$ and $\purpleD7$. It is not evenly divisible by any other whole numbers.
A factor is a whole number that can divide evenly into another number.
The factors of $\purpleD8$ are: $\blueD{1, 2, 4},$ and $\blueD8$.
$\purpleD8 \div \blueD1 = 8$
$\purpleD8 \div \blueD2 = 4$
$\purple8 \div \blueD4 = 2$
$\purpleD8 \div \blueD8 = 1$
$\blueD{1, 2, 4},$ and $\blueD8$ all divide evenly into $\purpleD8$.

## Practice set 1: Identifying prime numbers

Problem 1A
Which of these numbers is prime?
$\large{12,\ 42,\ 61,\ 87,\ 95}$

Want to try more problems like this? Check out this exercise.

## What are composite numbers?

Composite numbers have more than $2$ factors. A composite number has factors in addition to $1$ and the number $\text{itself}$.
$16$ is an example of a composite number. The factors of $16$ are $1, 2, 4, 8$ and $16.$ All of these numbers divide into $16$ evenly.
$\large{29,\ 41,\ 47,\ 82,\ 89}$